Study Sheds Light on Spread of Cruise Ship Norovirus Outbreaks

A study by the U.S. Centers for Disease Control and Prevention (CDC) revealed a cruise ship norovirus outbreak led to a 15 percent â€œattack rateâ€ despite early detection and â€œaggressiveâ€ measures, according to MedPageToday.

The January 2009 outbreak led to 236 of the 1,532 passengers falling ill, according to CDC questionnaires completed on the cruise, said Mary Wikswo, MPH, and colleagues at the agency, wrote MedPageToday. Of those sickened with norovirus, about 60 percent sought medical treatment on board, said the CDC in the online journal Clinical Infectious Diseases.

The researchers concluded that, perhaps low voluntary reporting could be due to the current threshold for pronouncing an outbreak. Today, the CDC’s Vessel Sanitation Program states â€œan outbreak of acute gastroenteritis is declared when at least three percent of passengers report illness,â€ wrote MedPageToday. The three percent threshold was met on so-called Cruise Ship X within three days of the two-week sail, according to the study group, said MedPageToday.

Norovirus outbreaks occur frequently in closed populations, such as on cruise ships, and are a group of viruses that cause swelling in the linings of both the stomach and intestines, according to the CDC. A highly contagious, severe gastrointestinal illness commonly referred to as the so-called â€œstomach flu,â€ Norovirus spreads quickly because it transmits easily through the vomit and feces of people sick with the illness. Contact with only a few particles can make a person ill.

Norovirus, which can survive for weeks on surfaces at room temperature, can be difficult to eliminate, and can only be killed with chorine bleach. Alcohol-based hand sanitizers and other preparations are not too helpful. People are generally considered to be contagious from when they feel ill to about three days after their symptoms subside; however, the virus can still be active in their vomit or stool for two weeks or more.

Weâ€™ve long been writing about the ongoing incidences of norovirus outbreaks on cruise ships, including one in January on a Royal Caribbean Radiance of the Seas with hundreds of ill passengers. Just prior, 96 people on the Sapphire Princess fell ill on a Princess Cruise ship. Over 400 passengers on a recent Celebrity Cruise lines trip were sickened in just one of eight prior such outbreaks in 2010; four took place in one week, said the CDC, which runs a cruise ship sanitation program that was criticized in 2009 for doing a poor job of detecting dirty cruise ship bathrooms. In 2009, there were a total of 15 cruise ship outbreaks of Norovirus. And, according to a prior New York Times article, there were more than 60 outbreaks of Norovirus on cruise ships since 2005.

For its study, CDC investigators boarded the vessel, taking passenger samples, testing sanitation protocols, and administering questionnaires to all passengers, said MedPageToday; 83.2 percent of the passengers completed the questionnaire. Because of the low incidence of illness among the crew, they were excluded.

Among other findings, the team felt the outbreak could have originated with an ill passenger whose contaminated vomit could have been ingested when particles went airborne or could have contaminated surfaces. All eight episodes of vomiting in public areas were cleaned within a half-hour; however, the researchers found that the absorbing material used to clean up was not used as intended, said WebPageToday. The CDC said that “cruise line personnel should discourage ill passengers from boarding their ships” and “passengers and crew who become ill should report to the ship’s medical center as soon as possible,” quoted MedPageToday.

Several violations could have contributed to the outbreak, such as toilets not being correctly stocked with soap and towels and dishwashing machines that did not appropriately clean utensils, said MedPageToday.